An avian egg, in general, comprises an eggshell, two eggshell membranes, and an egg white and an egg yolk. Both the egg white and the egg yolk contain nutritionally physiologically valuable components such as proteins, for instance ovalbumin, ovotransferrin, ovomucin, and others, as well as lipids. The egg white is known as the albumen (Latin, albus, for “white”) and is comprised of four alternating layers of thick and thin albumen contain approximately 40 different proteins, which are the main components of the egg white in addition to water. The egg white is approximately two-thirds of the total egg's weight out of its shell with 90% of that weight coming from water. The remaining weight of the egg white comes from protein, trace minerals, fatty material, vitamins, and glucose. The U.S. large egg's white typically weighs 38 grams with 3.9 grams of protein, 0.3 grams of carbohydrate and 62 milligrams of sodium. The most predominant proteins and their approximate respective percentage of composition of the albumen include the following: Ovalbumin (54%), Ovotransferrin (12%), Ovomucoid (11%), Globulins (8%) (Function-Plugs defects in membranes, shell), Lysozyme (3.5%), Ovomucin (1.5%), Avidin (0.06%), and others (10%). There are also opaque ropes of egg white, called the chalazae, that hold the yolk in the center of the egg. They attach the yolk's casing to the membrane lining the eggshell. Additionally, there is a vitelline membrane, a clear casing that encloses the yolk.
The egg yolk makes up about 33% of the liquid weight of the egg, a majority of the calories contained in the egg, most of the minerals (iron, phosphorus, calcium, thiamine, and riboflavin) and virtually all of the fat soluble vitamins (A, D, E and K). The approximate composition (by weight) of the most prevalent fatty acids in egg yolk include: unsaturated fatty acids (Oleic acid (47%), Linoleic acid (16%), Palmitoleic acid (5%), Linolenic acid (2%)) and saturated fatty acids (Palmitic acid (23%), Stearic acid (4%), Myristic acid (1%)). The yolk is also a source of lecithin, a common emulsifier.
Each of the various egg components has utility in a variety of industries. Specifically, the egg component's utility spans across the food, nutraceutical, pharmaceutical and cosmetic industries. Egg powders are utilized in a variety of food applications. For example, egg yolk and whole egg powders and egg albumen powders are used in a variety of baked goods, mayonnaise, quiches and other food applications requiring whipping or binding of egg products. Additionally, the substitution of egg powders allows manufacturers to add additional nutritional value to the products. In nutraceutical, cosmetic and pharmaceutical applications, lysozyme, avidin and ovotransferrin can be extracted from the egg albumen. Such products can be utilized as natural antimicrobial (lysozyme), food preservatives in cheese and wine, a nutritional ingredient in iron-fortified products (Ovotransferin), as well as use in pharmaceuticals. Additionally, avidin may be used in biotechnology research for diagnostic kits.
An individual analysis of some of the egg's components follows, including some of particular commercial interest.
Ovotransferrin is a neutral glycoprotein synthesized in the hen oviduct and deposited in the egg white albumen at a ratio of approximately 12% of the total protein content. Ovotransferrin is an 80 kDa matrix covalent dimmer protein observed at high concentration in the uterine fluid at the initial stage of shell mineralization and is also present in extracts from demineralized eggshell, the sites of calcite nucleation. Northern blotting and RT-PCR demonstrate that ovotransferrin is expressed in the proximal oviduct, and at a lower magnitude in the distal oviduct. Ovotransferrin is also present in the tubular gland cells of the uterus. Ovotransferrin is also thought to impact calcium carbonate crystals and calcite morphology, suggesting that ovotransferrin has a dual role, a protein influencing nucleation and growth of calcite crystals and as a bacteriostatic filter to reinforce its inhibition of Salmonella growth in egg albumen.
Ovotransferrin can be used as a nutritional ingredient in iron-fortified products such as iron supplements, iron fortified mixes for instant drinks, sport bars and protein supplements and iron-fortified beverages. There is also extensive evidence of an antibacterial effect of ovotransferrin based on iron deprivation, iron being an essential growth factor for most micro-organisms. Ovotransferrin tightly binds transition metals (Fe[III], Cu[III], Al[III]) with a binding log constant of about 15 at pH 7.0 and higher. In vivo, ovotransferrin demonstrates therapeutic properties against acute enteritis in infants (Corda, R., et al., Conalbumen in the treatment of acute enteritis in the infant, Int. J. Tiss. Reac. V(1), 117-123 (1983)).
Ovalbumin is the main protein found in egg white, making up close to 60% of the total protein. The ovalbumin functions as nourishment and blocks digestive enzymes. It belongs to the serpin superfamily of proteins, although unlike the majority of serpins it is unable to inhibit any proteases. The ovalbumin protein of chickens is made up of 385 amino acids, and its relative molecular mass is 45 kDa. It is a glycoprotein with 4 sites of glycosylation. It is secreted from the cell, despite lacking an N-terminal leader sequence. The function of ovalbumin is unknown, although it is presumed to be a storage protein.
Ovalbumin is useful in cases of poisoning by heavy metals (such as iron) as a chelator to heavy metals by trapping the metal ions within the sulfhydryl bonds of the protein and preventing the absorption of the metals into the gastrointestinal tract and prevents poisoning. Additionally, it is an important protein in several different areas of research. Ovalbumin is commonly used in general studies of protein structure and properties. It is also utilized in studies of serpin structure and function (the fact that ovalbumin does not inhibit proteases means that by comparing its structure with that of inhibitory serpins, the structural characteristics required for inhibition can be determined). Ovalbumin is also used in proteomics, as a molecular weight marker for calibrating electrophoresis gels. Additionally, ovalbumin is utilized in immunology studies as a stimulator of allergic reactions in test subjects.
Lysozyme is a 14.4 kDa, 129 aminoacid residue enzyme that damages bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between N-acetyl-D-glucosamine residues in chitodextrins. Large amounts of lysozyme can be found in egg whites and the eggshell, as originating from the uterine fluid. Lysozyme protein is highly concentrated in the limiting membrane circumscribing the egg white and forms the innermost layer of the shell membranes. It is also present in the shell membranes, and in the matrix of the calcified shell. Lysozyme provides anti-microbial protection as it digests bacterial cell walls and protective structural properties to an eggshell.
Lysozyme functions by attacking peptidoglycans (found in the cells walls of bacteria, especially Gram-positive bacteria) and hydrolyzing the glycosidic bond that connects N-acetylmuramic acid with the fourth carbon atom of N-acetylglucosamine. It does this by binding to the peptidoglycan molecule in the binding site within the prominent cleft between its two domains. This causes the substrate molecule to adopt a strained conformation similar to that of the transition state. The lysozyme binds to a hexasaccharide and then distorts the 4th sugar in hexasaccharide (the D ring) into a half-chair conformation. In this stressed state the glycosidic bond is easily broken. The amino acid side chains glutamic acid 35 (Glu35) and aspartate 52 (Asp52) have been found to be critical to the activity of this enzyme. Glu35 acts as a proton donor to the glycosidic bond, cleaving the C—O bond in the substrate, whilst Asp52 acts as a nucleophile to generate a glycosyl enzyme intermediate. The glycosyl enzyme intermediate then reacts with a water molecule, to give the product of hydrolysis and leaving the enzyme unchanged. The amino acid side chains glutamic acid 35 (Glu35) and aspartate 52 (Asp52) have been found to be critical to the activity of this enzyme. Glu35 acts as a proton donor to the glycosidic bond, cleaving the C—O bond in the substrate, whilst Asp52 acts as a nucleophile to generate a glycosyl enzyme intermediate. The glycosyl enzyme intermediate then reacts with a water molecule, to give the product of hydrolysis and leaving the enzyme unchanged.
Lysozyme protein is abundant in the limiting membrane that circumscribes the egg white and forms the innermost layer of the shell membranes. It is also present in the shell membranes, and in the matrix of the calcified shell. Calcite crystals grown in the presence of purified hen lysozyme exhibited altered crystal morphology. Therefore, in addition to its well-known anti-microbial properties that could add to the protective function of the eggshell during embryonic development, shell matrix lysozyme may also be a structural protein which in soluble form influences calcium carbonate deposition during calcification.
Lysozyme is used in the food industry due to its ability to selectively inhibit the uncontrolled growth of Clostridium tyrobutyricum during the maturation of cheeses. Additionally, lysozyme can be used to protect against bacterial, viral or inflammatory diseases. It can be used as an aerosol for the treatment of bronchopulmonary diseases and for its prophylactic function against infectious pathogens of the buccal cavity, such as dental caries. It can further be used in droplets for nasal tissue protection and various therapeutic creams designed for the protection and topical reparation of certain diseases such as Herpes and shingles, as well as the treatment of recurrent aphthous stomatitis. Oral administration of lysozyme has also been shown to have immunostimulation effects in addition to antihistamine effects.
Various proteins are contained within eggs, including TGF-β and IgF-1. Transforming growth factor beta (TGF-β) is a protein having three isoforms (TGF-β1, TGF-β2 and TGF-β3). It is synthesized in a wide variety of tissues. The TGF-β family is part of a superfamily of proteins known as the transforming growth factor beta superfamily, which includes inhibins, activin, anti-müillerian hormone, bone morphogenetic protein, decapentaplegic and Vg-1. TGF-β controls proliferation, differentiation, and other functions in most cell types. It can also act as a negative autocrine growth factor. TGF-β induces apoptosis in numerous cell types and therefore plays a crucial role in the regulation of the cell cycle.
Insulin-like growth factors (IGFs) are polypeptides with high sequence similarity to insulin. They are part of a complex system cells use to communicate with their physiologic environment, consisting of two cell-surface receptors (IGF1R and IGF2R), two ligands (IGF-1 and IGF-2), a family of six high-affinity IGF binding proteins (IGFBP 1-6), as well as associated IGFBP degrading enzymes, known as proteases. Most cells are affected by IGF-1, especially cells in muscle, cartilage, bone, liver, kidney, nerves, skin, and lungs. In addition to the insulin-like effects, IGF-1 can also regulate cell growth and development, especially in nerve cells, as well as cellular DNA synthesis. IGF-2 is secreted by the brain, kidney, pancreas and muscle in mammals and birds.
Transfer factors are immune messenger molecules found in all higher animals. They are found in white blood cells, colostrum, and eggs. They transfer immunity against many pathogens that would otherwise kill the offspring of a species. They derive from leukocyte lysates of immune donors which can transfer strong local and systemic cellular immunity to non-immune recipients. Transfer factors could be utilized to immunity between compatible sources. They can be utilized by one prone to illness, i.e., colds, sore throats, ear infections, influenza, and numerous other ailments and diseases, in place of conventional commercially available immune boosters, preventions or treatments.
Sialic acids are a group of naturally occurring N- and O-acyl derivatives of the deoxyamino sugar neuraminic acid. Sialic acid consists of acetylated, sulfated, methylated, and lactylated derivatives and is a large family of more than 50 members. They are ubiquitously distributed in many animal tissues and in bacteria, primarily in glycoproteins and gangliosides. They are typically the terminal residues on cell surface oligosaccharides. Sialic acid-rich glycoproteins bind selectin in humans and other organisms. Sialic acid is also naturally occurring in eggs.
Sialic acid is useful as a pre-cursor to many anti-inflammatory medications. Various pharmaceutical agents and diagnostic reagents for influenza viruses are used in medical fields, leading to an increased demand for larger amounts of sialic acid worldwide.
Arachadonic acid is an omega-6 fatty acid, unsaturated, essential fatty acid, found in egg yolks. Arachadonic acid is a desirable product, because unlike the bad fats such as saturated fatty acids and cholesterol, arachadonic acid is a good fat, essential to stay healthy. Essential fatty acid deficiencies can lead to reduced growth, inability to fight infections and infertility. Although arachadonic acid deficiencies are not extremely common in the United States, they do exist in greater prevalence throughout the world. Arachadonic acid is needed to strengthen cell membrane integrity, repair cellular and tissue damage, optimize neurological transmission and brain function, improve heart and circulatory function and produce supple, moist skin. Therefore, obtaining a source of arachadonic acid from eggs could be used in numerous forms to provide supply for the human body to use to synthesize regulatory molecules such as prostaglandins (hormone like chemical messenger) and thromboxanes (involved in platelet aggregation and blood clotting).
Lipoproteins are globular, micelle-like particles that consist of a non-polar core of acylglycerols and cholesteryl esters, surrounded by an amphiphilic coating consisting of protein, phospholipid and cholesterol. Lipoproteins have been classified into five broad categories on the basis of their functional and physical properties: chylomicrons (which transport dietary lipids from intestine to tissues), very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low density lipoproteins (LDL), (all of which transport triacylglycerols and cholesterol from the liver to tissues), and high density lipoproteins (HDL) (which transport endogenous cholesterol from tissues to the liver).
Lipoprotein particles undergo continuous metabolic processing and have variable properties and compositions. Lipoprotein densities increase without decreasing particle diameter because the density of their outer coatings is less than that of the inner core. The protein components of lipoproteins are known as apolipoproteins. High concentrations of lipoproteins in the diet are strongly associated with increased risk of cardiovascular disease, including increased risk for atherosclerosis and its manifestations, which include hypercholesterolemia, myocardial infarction, and thrombosis.
Carotenoids are naturally occurring organic pigments. Carotenoids are found in abundance (over 600 types) in nature and are divided into two classes: xanthophylls and carotenes. The xanthophylls include lutein and zeaxanthin, both naturally occurring in eggs. The yellow color of chicken egg yolks is a result of ingested xanthophylls. In fact, eggs are one of the richest natural resources containing carotenoids. Carotenoids have a number of important physiological properties and are often formulated into a variety of therapeutic drugs or nutritional supplements.
The carotenoids, lutein and zeaxanthin have a yellow color, as seen in an egg yolk, because they absorb the high-energy radiation of the near-ultraviolet and blue light spectrum and reflect the yellow/yellow-orange wavelengths. It is theorized that since these two pigments absorb wavelengths in the high-energy spectrum, they may help protect retinal cells in the macula against “phototoxic” damage caused by short-wavelength high-energy light radiation. Additionally, lutein and zeaxanthin are chemically very closely related to each other; both have the exact same chemical formulae, differing only in their ring stereochemistry and the spatial placement of one end ring and the placement of a double bond in that end ring
Lutein is classified as a carotenoid and is one of many nutrients naturally occurring in egg yolks, and in part provides the yellow color of the egg yolk. It has a variety of important physiological properties, including prevention of age-related macular degeneration, a gradual worsening of vision due to degeneration of a portion of the retina and one of the leading causes of blindness, as well as decreasing the development of cataracts within the eye. Lutein assists in protecting eyes from oxidative stress and high-energy light damage. Although there are no daily recommended amounts of lutein, there is a demand for lutein in many diets as individuals seek to increase lutein consumption in their diets through lutein-fortified foods, sublingual sprays and dietary supplements, including both nutraceuticals and pharmaceuticals. Additionally, lutein is utilized within pet food markets and human skin care and cosmetics.
Zeaxanthin is a carotenoid found in egg yolks providing its yellow color. It can be used as a feed additive, a colorant in the cosmetic and food industries, and dietary supplements. Zeaxanthin has a variety of important physiological properties, including prevention and treatment of age-related macular degeneration, to which there is no cure and treatments are currently limited, as well as decreasing the risk of cataract development. Other carotenoids, such as beta-carotene, vitamin A, and vitamin E have generally been used and seen as beneficial anti-oxidant that may slightly retard the rate of macular degeneration. However, they fail to rise to the level of truly effective treatments. Zeaxanthin is not a widely available chemical, and is not available to the public except in extremely small trace quantities in mixtures of other less beneficial carotenoids.
Typical methods of separating components from avian eggs include diluting the egg yolks and whites with water at a pH from about 4-5 so that the lipids separate out of solution. One disadvantage of this dilution technique can be its lack of efficiency since the lipids have to settle out of solution overnight on their own. Centrifugation is not possible as it would stir up the lipids and hinder the separation of proteins from the solution. Further, with this type of processing, it can be difficult to obtain a high yield of proteins. The known technique is also disadvantageous in that only one protein of the egg, IgY, is isolated and the rest of the egg material discarded, rendering the process time-consuming, economically inefficient, and wasteful.
Thus, although it is generally recognized that eggs contain numerous nutritionally and physiologically or otherwise valuable components, problems remain, particularly with respect to recovery of such components in efficient and cost-effective ways without damaging the components or compromising their suitability of the components for use in nutritional, medicinal, pharmaceutical, health, cosmetic or related uses.